1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser. The improvement is for obtaining a laser with a higher yield.
2. Description of the Prior Art
In developments of the semiconductor laser, for example, (GaAl)As lasers are now actively being made, and such a long lifetime as 10.sup.5 to 10.sup.6 hours have been already actually achieved for (GaAl)As lasers for 0.8 .mu.m (wave length range).
For use in video disk players and laser printers, or the like information processing apparatus, now visible light lasers of (GaAl)As or Ga(AsP) or GaInAsP which oscillate in 0.7 .mu.m (wave length range) are now actively developed, and especially the (GaAl)As visible light laser is now in the stage of practical use.
Reported minimum wavelength of the (GaAl)As is about 710 nm, and for the one of shorter wavelength the lifetime of the laser becomes very much short. One of the supposed reasons for such short lifetime is undesirable diffusions of impurity in the p-type clad layer into the active layer during the epitaxial growth step in the manufacture or in the operation of the laser, as well as, resultant forming of in non-uniform carrier distribution in the active layer and also consequent forming of dark spots and dark lines to form defects therein. Therefore, it is desirable to adopt such an impurity for the clad layers that is not likely to diffuse into the active layer. The impurities Sn and Ge respectively as donor and acceptor for clad layers of the (GaAl)As infrared laser has desirably small vapor pressures and also has desirably low diffusion thereof into neighboring layer and hence are usable to obtain concentration of 5.times.10.sup.17 cm.sup.-3 for such infrared laser, and therefore Sn and Ge have been used in the infrared laser. However, impurities Sn and Ge are not suitable for the visible light (GaAl)As laser, since in the p-type and n-type layer the attainable highest impurity concentration with these impurities is only 10.sup.16 cm.sup.-3 when the Al fraction x.times.0.6-0.7 in the clad layers, and therefore the clad layers become of high resistivity, and hence poor yield of production.
Because of the abovementioned reason, Te and Zn are used for the donor and acceptor for the clad layers of the visible light (GaAl)As lasers and its doping amount is adjusted so as to obtain 5.times.10.sup.17 cm.sup.-3. However, such conventional (GaAl)As visible light laser has had the problem that an extraordinarily careful controlling is required in the epitaxial growth process for manufacturing with a high yield in order to prevent undesirable diffusion of such impurities from the clad layers into the neighboring active layer due to the high vapor pressure of Te or Zn, resulting in high cost of manufacture.
One typical example of the abovementioned visible light (GaAl)As laser is elucidated referring to FIG. 1, wherein by means of known liquid phase epitaxial growth method, on
______________________________________ a substrate 1 of n-GaAs the following layers are sequentially formed: a first clad layer 2 of Te--doped n-Ga.sub.1-x Al.sub.x As (2-3 .mu.m thick), an active layer 3 of non-doped Ga.sub.1-y Al.sub.y As (about 0.1 .mu.m thick), a second clad layer 4 of Zn--doped p-Ga.sub.1-x Al.sub.x As (about 1.5 .mu.m thick), and a cap layer 5 of Ge-- or Zn--doped p.sup.+ -GaAs (0.5 .mu.m thick). ______________________________________
Thereafter, electrodes 8 and 9 are formed.
The Al fractions x and y are selected to be x=0.3-0.85 and y=0-0.25 for oscillation of the wavelength of 9000 to 7000 A.
In the conventional laser shown in FIG. 1, the p-type clad layer 4 contacts the active layer 3 with all the area thereof, and therefore, the impurity Zn in the clad layer 4 considerably diffuses into the active layer, and accordingly the characteristic and lifetime of the laser is influenced by the diffusion. That is, as a result of the Zn-diffusion into the active layer, the highest usable temperature becomes lower and the current-light output curve is likely to produce a kink as shown in FIG. 2, and the lifetime becomes 1/5 times as that of the laser having Ge-doped clad layers.